Mechanical aerator

ABSTRACT

A mechanical aerator for the aeration of a liquid, having an impeller positioned so as to contact the surface of the liquid, the impeller comprising: a shaft rotatable about a substantially vertical axis; and a plurality of backward curved vanes extending outwardly from the shaft, the vanes curved such that as the impeller rotates about a substantially vertical axis a series of thin film waves eminate spirally outward and upward from the vanes, such that each particle of liquid contained therein moves radially outward from the axis of rotation.

' Dec. 5, 1972 United States Patent OTHER PUBLICATIONS Weber, Selecting Turbine Agitators, December 7,- 1964; p. 169. from Chemical Engineering Magazine.

[54] MECHANICAL AERATOR [72] Inventor: Frank G. Weis, Kansas City, Mo. [73] Assignee: Ecodyne Corporation [22] Filed:

Sept. 25, 1970 Primary Examiner-Tim R. Miles Assistant ExaminerSteven H. Markowitz [211 App]' Attorney-Charles M. Kaplan and Joel E. Siegel [57] ABSTRACT V A mechanical aerator for the aeration of a liquid, hav- [52] U.S. Cl. [51] Int. Cl......

3/04, BOlf 7/20 Field of Search ........2l0/l97, 200; 259/7, 9, 95,

ing an impeller positioned so as to contact the surface 9 261/91 93 of the liquid, the impeller comprising: a shaft rotatable about a substantially vertical axis; and a plurality of backward curved vanes extending; outwardly from the shaft," the vanes curved such that as the impeller rotates about a substantially vertical axis a series of thin film waves eminate spirally outward and upward [56] References Cited UNITED STATES PATENTS 3,576,316 4/1971 Kaelin 2,054,395 9/1936 Streander..............................

from the vanes, such that each particle of liquid contained therein moves radially outward from the axis of 3,235,877 2/1966 Grob 3,341,450 9/1967 Ciabattar et al......

2,802,647 8/1957 B0|t0n.................. 1,497,425 6/l924 Ames et 8 Claims, 4 Drawing Figures FOREIGN PATENTS OR APPLICATIONS 3/196] GreatBritain........................ .,261/9 l 1 MECHANICAL AERATOR BACKGROUND OF TI-IEINVENTION The present invention is concerned with aeration apparatus and in particular with mechanical surface aeration apparatus employed in the treatment of sewage by the activated sludge process.

The aeration of sewage is usually accomplished in an aeration tank; and two types of aeration apparatus have been used quite extensively in the past. A firsttype of aeration apparatus makes use of plates, or other bodies of porous material, through which air is forced under substantial pressure. This type requires very considerable amounts of power and is subject to the serious operational disadvantage that the plates, or other porous material, become clogged during use. The other type of aeration apparatus is of the mechanical agitation type, and involves the useof a power-driven surface mechanical aerator. This type aerator generally includes a rotor carrying a plurality of vanes or blades. The rotor is mounted at the surface of the liquid in the aeration tank with its blades partially submerged in the liquid. The surface mechanical aerator is particularly desirable in that its operation is inherently very reliable and not subject to changes over its operative life. Also, it is very much less expensive than the porous plate arrangements both as to original cost and as to maintenance.

The prior art mechanical aerators generally include a shaft rotatable about a vertical axis, and a plurality of radial vanes either slanted or straight in circumferential attack on the liquid. These aerators have a very serious drawback in that they cause theliquid in the tank to rotate around a vertical axis in the same direction as the vanes move, thereby preventing the rotating vanes from imparting maximum horsepower to the liquid. This rotational movement of the tank contents also results in stratified liquid zones in a shape resembling a doughnut. The particles in these zones continually rotate around the tank in such amanner that the oxygen level is never changed, nor are these particles brought near the oxygenproducing source. The contents on the bottom of the tank circulate at a slow rate and the solids are never lifted from this spot. Further,

these aerators throw out the'liquid in coarse droplets and since the oxygen transfer from the air can only be at the surface of the droplet, the oxygen transfer is thus seriously limited.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an improved method and apparatus for aerating liquid.

Another object is to provide mechanical surface aeration apparatus that transfers oxygen to the liquid at greater eff ciency in terms of pounds of dissolved oxygen transferred in relation to driving horsepower than other aerators of this type.

A further object is to provide aerator apparatus wherein. the discharge of the liquidthrown out by the impeller does not cause rotation of the liquid within the tank around a vertical axis.

A still further object of the present invention is to provide an aerator apparatus wherein the liquid thrown out from the impeller is substantially in thin film waves, so as to result in greater oxygen transfer.

Another object is to provide an aerator apparatus for use in the treatment of sewage by the activated sludge process that agitates the mixture of sludge and sewage so as to maintain the sewage in suspension and in continuous contact with oxygen.

Accordingly, the present invention provides an aerator of the paddle type with an open impeller having backward curved vanes. The vanes are curved in such a manner that the discharge of each particle of liquid thrown out by the impeller is radial from the axis of rotation of the impeller. The discharge may vary slightlyfrom'being radial, but not enough to cause rotation of the tank contents around avertical axis. By use of the curved vanes, the liquid is thrown out from the impeller in a series of thin film waves spiraling outward and upward towards the sides of the tank, with each particle of liquid within the waves :moving radially outward from the axis of rotation of the impeller. The thin filmwaves stretch out as they move outward from the impeller thereby continuously renewing the film surface particles, thus exposing new particles to the air and increasing oxygen transfer to all the liquid within the'waves.

Other and more specific objects of the invention and various features of the invention will be made apparent in the following description of certain preferred embodiments, and in the accompanying drawings wherein:

DESCRIPTION OF THE DRAWINGS FIG. 1 is an elevational view,'partially in cross section, which illustrates improved aeration apparatus embodying various features of the present invention;

FIG. 2 is an enlarged view of the impeller portion of the aeration apparatus shown in FIG. 1;

FIG. 3 isa plan view of the impeller, as in FIG. 2, showing the manner in which the liquid is discharged from the impeller vanes; and I FIG. 4 illustrates a development of one of thevanes employed in the impeller shown in FIGS. 2 and 3.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, aeration system 10 includes an aeration tank 11 into which sewage is introduced through inlet 13 and removed through outlet 15. The tank 11 may be round, square, or rectangular in horizontal cross section, and its volume generally determines the size or capacity of the aerator used. Support bridge 17 extends across tank 11 and supports electric motor 19 and gear reducer 21 thereon. Vertical shaft 23 extends downwardly from gear reducer 21 and is connected to impeller 30 at its lowermost end.

The present invention is particularly concerned with the design of impeller 30. As seen in FIGS. 2 and 3, impeller 30 is of the open paddle type having a plurality of circumferentially spaced backward curved rigid vanes 32 extending outwardly from shaft 23. It is the unique I design of vanes 32 which result in the high efficiency of R (DRs)/2.83, where D is the diameter of impeller 30 in inches, and Rs is the radius of shaft 23 in inches.

Each vane 32 is generated from an axis of generation passing through the center of shaft 23 spaced 3'60/n degrees from the axis of generation of the adjacent vane, where n is the number of vanes in the particular impeller 30, at a distance R from the center of shaft 23.

Although the optimum shape of vanes 32 is as mentioned immediately above, some variations in this particular curvature are also suitable in the practice of the present invention. The present invention contemplates the use of similarly curved vanes which are shaped so as to fall within an area defined between first and second arcs having radii of curvature R and R respectively, as seen in FIG. 4. The arcs are generated from a point located on an axis of generation extending radially outward from the center of shaft 23 a distance equal to R (i.e., (DRs)/2.83 from the center of shaft 23. From this point the arcs are generated having the following radii of curvature:

where D is thediameter of the impeller in inches and Rs is the radius of shaft 23 in inches. I 1

The optimum number (n) of vane 32 is dependent upon the diameter of impeller 30. For example, it has been found that a 36 inch diameter impeller functions best with 4 vanes, whereas a 72 inch diameter impeller functions best with 6 vanes. Likewise, the width (w) of each vane 32 varies dependent upon the diameter of the impeller 30; i.e.,' a 36 inch diameter need only be 4 inches wide whereas a 72 inch diameter impeller should be 6 inches wide. The thickness of all vanes 32 for impellers 30 inches up to 72 inches diameter is three-eighths inch, and one-half inch for all impellers having a larger diameter.

In operation, a mixture of sewage and sludge enters tank 11 through inlet 13 and exits through outlet 15. Impeller 30 is preferably positioned in tank 11 such that its upper surface is adjacent the surface of the liquid in the tank. For example if the vanes are 6 inches wide it is positioned such that the bottom surface of the vanes are 6 inches below the surface of the liquid prior to initiating rotation. Motor 19 supplies the energy to rotate impeller 30, via gear reducer 21 and shaft 23, in

a counter-clockwise direction.

As impeller 30 rotates about a vertical axis vanes 32 cut through the liquid causing a quantity of liquid to be gauged from the tank contents. The curved vanes then gently loft liquid into a series of thin film waves eminating spirally outward and upward towards the tank sidewalls, as illustrated in FIG. 3. These thin film waves Stretch out as they move outward from impeller 30 thereby continuously renewing the surface particles exposed to the atmosphere. This results in a low oxygen gradient across the film thereby increasing the oxygen transfer. Further, a secondary result is a high oxygen transfer at the liquid surface-air interface caused by the agitation of the falling cascades of the series of films.

Another feature of the liquid discharge from impeller 30 is that each particle of liquid within each film is moving in a direction substantially radially outward from the center of shaft 23, as indicated by the arrows in FIG. 2. Thus no component of motion is transmitted to the tank contents that would cause the contents to rotate around a vertical axis. This enables vanes 32 to impart maximum energy to the tank contents. Removal of this rotational motion also prevents the formation of Stratified liquid zones which continually rotate. around the tank such that the particles within such zones are not brought in contact with the oxygen producing source.

Impeller 30 circulates the tank contents by rotating them around ahorizontal axis as indicated by the arrows in FIG. 1. This circulation prevents the formation of dead pockets where no aeration takes place and solids settle. The liquid and solids are drawnup from the bottom of tank 11 by impeller 30 as a rising vertical column so as to throughly mix the contents of tank 11.

This is accomplished with out the use of draft tubes found in many prior art aerators.

Some of the advantages'of the present invention will become more apparent by a brief discussion of the following example. An aeration system including a steel test tank 26 feet 1 inch 26 feet 1 inch, was filled to a 9 foot depth with water from a creek in Lenexa, Kansas. The temperature of the water was 15C and it had a saturation concentration of 8 miligrams of oxygen per liter. A motor rated at 10 horsepower and 860 r.p.m. was connected to a speed reducer which caused a 2%inch diameter (2Rs),shaft to rotate at 72 r.p.m. Attached to the shaft was an impeller, as described herein, having inch thick arcuate-shaped cold rolled steel vanes having a radius of curvature (R) of 22 inches and a diameter (D) of inches. The width of the impeller was 6 inches and was submerged in the test water such that the uppermost portion thereof was at the top surface of the tank contents.

During operation of the above-described aeration system it was observed that the impeller discharged a series of thin films spirally outward and upward therefrom, which stretched out as they moved outward before returning to the surface of the tank contents in the form of a splashing movement approximately midway between the tip of the impeller and the tank sidewalls. It was further observed that each particle of water within the waves was moving radially outward from the axis of rotation of the impeller. This was further evidenced by stopping the rotation of the impeller and observing the movement of the tank contents radially outward from the center of the tank, and no rotation of either solids or liquids around a vertical axis occured after the impeller ceased rotation. In prior.

art systems where the tank contents are rotated around a vertical axis the momentum of the system causes the liquid and solids in the tank to continue to rotate around such vertical axis after the prior art impeller has stopped.

In order to determine the oxygen transfer rate of the system a Precision Scientific D. 0. Probe was positioned about 6 inches below the surface of the water at the tank perimeter. The chart immediately below shows the milligrams of dissolved oxygen per liter of water as a function of the length of time the system was in operation.

TIME (minutes) 2 3 4 5 3.8 4.7

D.O. 5.5 6.2 7.6v

The invention thus provides an improved aeration apparatus and method which produces a significant increase over other aerators ofthis type in the efficiency of oxygen transfer in terms of pounds of dissolved oxygen transferred in relation to driving horsepower.

While the embodiment described herein is at present considered to be preferred, it is understood that various modifications and improvements may be made therein, and it is intended to cover in the appended claims all such modifications and improvements as fall within the true spirit and scope of the invention.

What is claimed is:

l. The method of aerating a liquid confined within a container so as to increase oxygen transfer and prevent rotation of the liquid within the container, comprising:

a. immersingan impeller, having a plurality of elongated substantially horizontal open backward curved vanes, into the liquid at below the surface; and

b. rotating the impeller about a substantially vertical axis so as to generate a series of thin film waves eminating spirally outward and upward from the convex surface of the impeller vanes, such that each particle of liquid within the waves moves radially outward from the axis of rotation of the impeller.

2. In a paddle type aerator of the type used for the aeration of sewage, having an impeller positioned so as to contact the surface of the sewage, said impeller comprising:

a. a shaft rotatable about a substantially vertical axis;

and

b. a plurality of elongated substantially horizontal open backward curved vane means extending outwardly from said shaft, said vane means curved such that as said impeller rotates about a substantially vertical axis a series of thin film waves eminate spirally outward and upward from the convex surface of said vane means, such that each particle of sewage contained therein moves radially outward from said axis of rotation.

3. In a paddle type aerator of the type used for the aeration of a liquid, having an impeller positioned so as to contact the surface of the liquid, said impeller comprising:

a. a shaft rotatable about a substantially vertical axis;

and

b. a plurality of elongated substantially horizontal open backward curved vanes extending outwardly from said shaft, each of said vanes being contained within an area defined between first and second circular arcs, said first and second arcs both being generated from a point located on a reference line extending radially outward from said shaft and least partially spaced a distance equal to (DRs)/2.83 from the center of said shaft, said first are having a radius of curvature equal to (D -Rs)/2.4 and said second arc having a radius of curvature equal to (DRs)/3.2,

where D is the diameter of the impeller in inches and Rs is the radius of the shaft in inches. x 4. In a paddle type aerator of the type used for the aeration of a liquid, having an impeller positioned so as to contact the surface of the liquid, said impeller comprising:

a. a shaft rotatable about a substantially vertical axis;

b. a plurality of elongated substantially horizontal open backward curved vanes extending outwardly from said shaft, each of said-vanes being a portion of a cylinder whose radius of curvature is substantially equal to (D-Rs)/2.83 where D is the diameter of the'impeller in inches and Rs is the radius of the shaft in inches.

S. In a sewage treatment apparatus including a container having an inlet and an outlet adapted for flow of sewage therethrough, aeration means having an impeller positioned so as to contact the surface of the liquid, and means to rotate said impeller, said impeller comprising:

a. a shaft rotatable about a substantially vertical axis;

and r b. a plurality of circumferentially spaced elongated substantially horizontal open backward curved vane means extending outwardly from said shaft, said vane means curved such that as said impeller rotates about said substantially vertical axis the sewage particles that contact the convex surface of said vane means are thrown outward along a line substantially radial to said axis of rotation.

6. The invention defined in claim 5 wherein said impeller discharges a series of thin film waves spiralling outward and upward therefrom.

7. The invention defined in claim 6 wherein each of said vane means being contained within an area defined between first and second circular arcs, said first and second arcs being generated from a point located on a reference line extending radially outward from said shaft and spaced a distance equal to (DRs)/2.83 from the center of said shaft, said first are having a radius of curvature equal to (DRs)/2.4 and said second are having a radius of curvature equal to (DRs)/3.2, where D is the diameter of the impeller ininches and Rs is the radius of the shaft in inches.

8. The invention defined in claim 6 wherein each of said vane means being a portion of a cylinder whose radius of curvature is substantially equal to (DRs)/ 2.83, where D is the diameter of the impeller in inches and Rs is the radius of the shaft in inches. 

1. The method of aerating a liquid confined within a container so as to increase oxygen transfer and prevent rotation of the liquid within the container, comprising: a. immersing an impeller, having a plurality of elongated substantially horizontal open backward curved vanes, into the liquid at least partially below the surface; and b. rotating the impeller about a substantially vertical axis so as to generate a series of thin film waves eminating spirally outward and upward from the convex surface of the impeller vanes, such that each particle of liquid within the waVes moves radially outward from the axis of rotation of the impeller.
 2. In a paddle type aerator of the type used for the aeration of sewage, having an impeller positioned so as to contact the surface of the sewage, said impeller comprising: a. a shaft rotatable about a substantially vertical axis; and b. a plurality of elongated substantially horizontal open backward curved vane means extending outwardly from said shaft, said vane means curved such that as said impeller rotates about a substantially vertical axis a series of thin film waves eminate spirally outward and upward from the convex surface of said vane means, such that each particle of sewage contained therein moves radially outward from said axis of rotation.
 3. In a paddle type aerator of the type used for the aeration of a liquid, having an impeller positioned so as to contact the surface of the liquid, said impeller comprising: a. a shaft rotatable about a substantially vertical axis; and b. a plurality of elongated substantially horizontal open backward curved vanes extending outwardly from said shaft, each of said vanes being contained within an area defined between first and second circular arcs, said first and second arcs both being generated from a point located on a reference line extending radially outward from said shaft and spaced a distance equal to (D-Rs)/2.83 from the center of said shaft, said first arc having a radius of curvature equal to (D-Rs)/2.4 and said second arc having a radius of curvature equal to (D-Rs)/3.2, where D is the diameter of the impeller in inches and Rs is the radius of the shaft in inches.
 4. In a paddle type aerator of the type used for the aeration of a liquid, having an impeller positioned so as to contact the surface of the liquid, said impeller comprising: a. a shaft rotatable about a substantially vertical axis; and b. a plurality of elongated substantially horizontal open backward curved vanes extending outwardly from said shaft, each of said vanes being a portion of a cylinder whose radius of curvature is substantially equal to (D-Rs)/2.83 , where D is the diameter of the impeller in inches and Rs is the radius of the shaft in inches.
 5. In a sewage treatment apparatus including a container having an inlet and an outlet adapted for flow of sewage therethrough, aeration means having an impeller positioned so as to contact the surface of the liquid, and means to rotate said impeller, said impeller comprising: a. a shaft rotatable about a substantially vertical axis; and b. a plurality of circumferentially spaced elongated substantially horizontal open backward curved vane means extending outwardly from said shaft, said vane means curved such that as said impeller rotates about said substantially vertical axis the sewage particles that contact the convex surface of said vane means are thrown outward along a line substantially radial to said axis of rotation.
 6. The invention defined in claim 5 wherein said impeller discharges a series of thin film waves spiralling outward and upward therefrom.
 7. The invention defined in claim 6 wherein each of said vane means being contained within an area defined between first and second circular arcs, said first and second arcs being generated from a point located on a reference line extending radially outward from said shaft and spaced a distance equal to (D-Rs)/2.83 from the center of said shaft, said first arc having a radius of curvature equal to (D-Rs)/2.4 and said second arc having a radius of curvature equal to (D-Rs)/3.2, where D is the diameter of the impeller in inches and Rs is the radius of the shaft in inches.
 8. The invention defined in claim 6 wherein each of said vane means being a portion of a cylinder whose radius of curvature is substantially equal to (D-Rs)/2.83, where D is the diameter of the impeller in inches and Rs is the radius of the shaft in inches. 